1. Field of the Invention
The present invention relates to an amplification circuit which reduces drift of an input offset voltage using a chopper amplification circuit, and a current sensor which uses the amplification circuit.
2. Description of the Related Art
A chopper amplification circuit is widely used for a DC amplification circuit with high accuracy which reduces drift of an input offset voltage. A general chopper amplification circuit is configured by a switch circuit which modulates an input signal to a constant frequency, an amplifier which amplifies the modulated signal, and a switch circuit which demodulates the modulated signal amplified by the amplifier. Normally, a filter circuit for removing high frequency components included in a demodulated signal is provided in a rear stage of a chopper amplification circuit. In a case in which an input signal is directly amplified, drift components of an input offset voltage are also collectively amplified, and thus a large error due to the drift components is not generated in an output signal. Since the chopper amplification circuit converts an input signal into a modulated signal with a higher frequency than a frequency range of drift components, amplifies the modulated signal, and returns the modulated signal to a signal with an original frequency range by demodulating the modulated signal, and thus it is possible to sufficiently reduce the error due to the drift components.
The aforementioned chopper amplification circuit can be applied to a circuit for correcting an input offset voltage in an operational amplifier or the like having high accuracy and broadband. A chopper amplification circuit for amplifying the input offset voltage in a low drift is used for such an offset correction circuit. That is, negative feedback control is performed such that the input offset voltage amplified by the chopper amplification circuit approaches zero volts. The chopper amplification circuit is used exclusively for amplifying the input offset voltage close to a DC voltage, and the amplification of an input signal is performed by another fast amplifier. Hence, the response speed of an offset correction circuit, including the chopper amplification circuit, hardly influences frequency characteristics of the entire circuit during a normal operation state.
However, for example, in a case in which an excessive signal is input and an output is scaled out to a maximum level, or the like, a DC potential in a circuit can enter a saturation state greatly departed from a normal state. When the saturation state returns to the normal state, the response speed of an offset correction circuit becomes a problem. That is, since negative feedback control is not performed in the saturation state and the offset correction circuit operates temporarily and independently, the entire circuit cannot return to the normal state until the offset correction circuit independently returns to the normal state. Particularly, the response speed of a filter circuit provided in a rear stage of the chopper amplification circuit is delayed in the offset correction circuit, and thus recovery time for returning to the normal state becomes prolonged.